Dielectrophoretic displays

ABSTRACT

A dielectrophoretic display comprises a substrate having walls defining at least one cavity, the cavity having a viewing surface and a side wall inclined to the viewing surface; a suspending fluid contained within the cavity; a plurality of at least one type of particle suspended within the suspending fluid; and means for applying to the substrate an electric field effect effective to cause dielectrophoretic movement of the particles to the side wall of the cavity.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a non-provisional of Provisional ApplicationSerial No. 60/419,019, filed Oct. 16, 2002. This application is also acontinuation-in-part of copending application Ser. No. 08/983,404, filedMar. 26, 1999, which is the United States National Phase ofInternational Application No. PCT/US96/12000, filed Jul. 19, 1996, whichis itself a continuation-in-part of application Ser. No. 08/504,896,filed Jul. 20, 1995 (now U.S. Pat. No. 6,124,851). The entire disclosureof all the aforementioned applications, and of all U.S. patents andpublished applications mentioned below, is herein incorporated byreference.

BACKGROUND OF INVENTION

[0002] This invention relates to dielectrophoretic displays, and methodsfor operating such displays. More specifically, this invention relatesto dielectrophoretic displays in which solid particles and a suspendingfluid are held within a cavity.

[0003] Electrophoretic displays have been the subject of intenseresearch and development for a number of years. Such displays use adisplay medium comprising a plurality of electrically charged particlessuspended in a fluid. Electrodes are provided adjacent the displaymedium so that the charged particles can be moved through the fluid byapplying an electric field to the medium. In one type of suchelectrophoretic display, the medium comprises a single type of particlehaving one optical characteristic in a fluid which has a differentoptical characteristic. In a second type of such electrophoreticdisplay, the medium contains two different types of particles differingin at least one optical characteristic and in electrophoretic mobility;the particles may or may not bear charges of opposite polarity. Theoptical characteristic which is varied is typically color visible to thehuman eye, but may, alternatively or in addition, be any one of more ofreflectivity, retroreflectivity, luminescence, fluorescence,phosphorescence, or (in the case of displays intended for machinereading) color in the broader sense of meaning a difference inabsorption or reflectance at non-visible wavelengths.

[0004] Electrophoretic displays can be divided into two main types,namely unencapsulated and encapsulated displays. In an unencapsulatedelectrophoretic display, the electrophoretic medium is present as a bulkliquid, typically in the form of a flat film of the liquid presentbetween two parallel, spaced electrodes. Such unencapsulated displaystypically have problems with their long-term image quality which haveprevented their widespread usage. For example, particles that make upsuch electrophoretic displays tend to cluster and settle, resulting ininadequate service-life for these displays.

[0005] An encapsulated, electrophoretic display differs from anunencapsulated display in that the particle-containing fluid is notpresent as a bulk liquid but instead is confined within the walls of alarge number of small capsules. Encapsulated displays typically do notsuffer from the clustering and settling failure mode of traditionalelectrophoretic devices and provides further advantages, such as theability to print or coat the display on a wide variety of flexible andrigid substrates.

[0006] For further details regarding encapsulated electrophoreticdisplays, the reader is referred to U.S. Pat. Nos. 5,930,026; 5,961,804;6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851;6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,721;6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971;6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786;6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072;6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649;6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; and6,580,545; and U.S. patent applications Publication Nos. 2002/0019081;2002/0021270; 2002/0053900; 2002/0060321; 2002/0063661; 2002/0063677;2002/0090980; 2002/0106847; 2002/0113770; 2002/0130832; 2002/0131147;2002/0145792; 2002/0171910; 2002/0180687; 2002/0180688; 2002/0185378;2003/0011560; 2003/0011867; 2003/0011868; 2003/0020844; 2003/0025855;2003/0034949; 2003/0038755; 2003/0053189; 2003/0076573; 2003/0096113;2003/0102858; 2003/0132908; 2003/0137521; 2003/0137717; and2003/01151702; and International Applications Publication Nos. WO99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO 00/36560; WO00/67110; WO 00/67327; WO 01/07961; and WO 01/08241. All of thesepatents and applications are in the name of, or assigned to, theMassachusetts Institute of Technology (MIT) or E Ink Corporation.

[0007] Many of the aforementioned patents and applications recognizethat the walls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called “polymer-dispersed” electrophoretic display inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,392,786, at column 6, lines 44-63. See also theaforementioned U.S. patent application Publication No. 2002/0131147, andthe corresponding International Application PCT/US02/06393. Accordingly,for purposes of the present application, such polymer-dispersedelectrophoretic media are regarded as sub-species of encapsulatedelectrophoretic media.

[0008] A related type of electrophoretic display is a so-called“microcell electrophoretic display”, sometimes also called a “microcup”electrophoretic display. In a microcell electrophoretic display, thecharged particles and the suspending fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium (or substrate), typically a polymericfilm. See, for example, International Applications Publication No. WO02/01281, and published U.S. application No. 2002/0075556, both assignedto Sipix Imaging, Inc.

[0009] Hereinafter, the term “microcavity electrophoretic display” willbe used to cover both encapsulated and microcell electrophoreticdisplays.

[0010] One of the problems with electrophoretic displays is the limitedrange of colors which each pixel of the display can produce. The priorart describes two main types of electrophoretic media. One typecomprises a single type of electrically charged particle in a coloredmedium. This type of medium is only capable of producing two colors ateach pixel; either the color of the particles or the color of the mediumis seen. The second type of medium comprises two different types ofelectrically charged particles in an essentially uncolored medium; thetwo types of particles may differ in polarity of electric charge or havecharges of the same polarity but differ in electrophoretic mobility.Again, this type of medium is only capable of producing two colors ateach pixel, namely the colors of the two types of particles.

[0011] One approach to expanding the limited range of colors availablefrom conventional electrophoretic displays is to place an array ofcolored filters over the pixels of the display. For example, considerthe effect on a display comprising white particles in a black fluid ofplacing an array of color filters (say red, green and blue) over theindividual pixels of the display. Moving the white particles adjacentthe viewing surface of a pixel covered with a red filter would colorthat pixel red, whereas moving the white particles of the same pixeladjacent the rear surface of the display would render the pixel black.The main problem with this approach to generating color is that thebrightness of the display is limited by the pixelation of the colorfilter. For example, if a red color is desired, the pixels covered byred filters are set to appear red. whereas the pixels covered by greenand blue filters are set to appear dark, so that only a fraction of thedisplay surface has the desired color while the remaining portion isdark, thus limiting the brightness of any color obtained. A reflectivedisplay that was capable of three optical states (black, white and coloror black, white and transparent) would significant advantages in imagequality, cost and ease of manufacture.

[0012] Conventional electrophoretic displays rely upon movement ofelectrically charged particles in an electric field under electrostaticforces; the particles move along the lines of force of the electricfield. However, it is known that objects can be moved bydielectrophoretic forces, that is to say that dipoles induced in theobjects by a non-uniform electric field cause the particles to movetowards regions of higher field strength. See, for example, U.S. Pat.No. 4,418,346 to Batchelder which describes an apparatus for providing adielectrophoretic display of visual information. In this apparatus, a“bubble” of a fluid is moved through a second, immiscible fluid in astepwise manner by applying voltages to closely spaced electrodes, thebubble being visible against a visually contrasting background. Visualinformation is conveyed by the position of the bubble relative to thebackground. The patent suggests that a simple one-dimensional display ofthis type might represent the level of an analog signal by the positionof the bubble. However, since the movement involved is that of a bubbleagainst a contrasting background, such an apparatus does not appearcapable of displaying an arbitrary image.

[0013] It has now been realized that using a microcavity electrophoreticmedium in a dielectrophoretic display greatly simplifies the problem ofgenerating the heterogeneous electric field required by such a display,since differences between the dielectric constant and/or conductivitybetween the suspending fluid and the material surrounding the suspendingfluid (such as a polymeric binder in which the capsules are embedded, asdescribed in many of the aforementioned E Ink and MIT patents andpublications, or the substrate in which the cavities of a microcellelectrophoretic display are formed) will result in a heterogeneouselectric field which can be used to move the particles within thesuspending fluid against the side walls of the cavities, therebyproviding the display with a substantially transparent state.

SUMMARY OF INVENTION

[0014] Accordingly, this invention provides a dielectrophoretic displaycomprising:

[0015] a substrate having walls defining at least one cavity, the cavityhaving a viewing surface and a side wall inclined to the viewingsurface;

[0016] a suspending fluid contained within the cavity;

[0017] a plurality of at least one type of particle suspended within thesuspending fluid; and

[0018] means for applying to the substrate an electric field effectiveto cause dielectrophoretic movement of the particles to the side wall ofthe cavity.

[0019] This invention also provides a process for operating adielectrophoretic display, the process comprising:

[0020] providing a substrate having walls defining at least one cavity,the cavity having a viewing surface and a side wall inclined to theviewing surface; a suspending fluid contained within the cavity; and aplurality of at least one type of particle suspended within thesuspending fluid; and

[0021] applying to the substrate an electric field effective to causedielectrophoretic movement of the particles to the side wall of thecavity.

[0022] In the dielectrophoretic display of the present invention, thesuspending fluid may be substantially uncolored, and have suspendedtherein only a single type of particle.

[0023] At least some of the at least one type of particle may beelectrically charged, and in one form of a display containing suchelectrically charged particles, the suspending fluid may have suspendedtherein a first type of particle having a first optical characteristicand a first electrophoretic mobility, and a second type of particlehaving a second optical characteristic different from the first opticalcharacteristic and a second electrophoretic mobility different from thefirst electrophoretic mobility. The first and second electrophoreticmobilities may differ in sign, so that the first and second types ofparticles move in opposed directions in an electric field, and thesuspending fluid may be substantially uncolored. This type of displaymay further comprise a backing member disposed on the opposed side ofthe cavity from the viewing surface, at least part of the backing memberhaving a third optical characteristic different from the first andsecond optical characteristics. The backing member may comprise areashaving third and fourth optical characteristics different from eachother and from the first and second optical characteristics. Inpreferred forms of such a display, the backing member comprises areashaving red, green and blue or yellow, cyan and magenta colors, and thefirst and second optical characteristics may comprise black and whitecolors.

[0024] In the dielectrophoretic display of the present invention, thecavity may have a non-circular cross-section, preferably a polygonalcross-section, as seen from the viewing surface. The at least one typeof particle may be formed from an electrically conductive material, forexample a metal, carbon black or a doped semiconductor.

[0025] As already indicated, in the dielectrophoretic display of thepresent invention, the substrate may comprises at least one capsule wall(typically a deformable wall) so that the dielectrophoretic displaycomprises at least one capsule. For reasons explained in theaforementioned 2003/0137717, the capsules are preferably arranged in asingle layer. Alternatively, the substrate may comprise a continuousphase surrounding a plurality of discrete droplets of the suspendingfluid having the at least one type of particle suspended therein (i.e.,the display may be of the polymer-dispersed type), or may comprises asubstantially rigid material having the at least one cavity formedtherein, the substrate further comprising at least one cover memberclosing the at least one cavity, so that the display is of the microcelltype.

[0026] In the process of the present invention, the electric field maybe an alternating electric field. When the display is of the type inwhich at least some of the at least one type of particle areelectrically charged, and the suspending fluid has suspended therein afirst type of particle having a first optical characteristic and a firstelectrophoretic mobility, and a second type of particle having a secondoptical characteristic different from the first optical characteristicand a second electrophoretic mobility different from the firstelectrophoretic mobility, with the first and second electrophoreticmobilities differing in sign, so that the first and second types ofparticles move in opposed directions in an electric field, the processmay further comprise:

[0027] applying an electric field of a first polarity to the cavity,thereby causing the first type of particles to approach the viewingsurface and the cavity to display the first optical characteristic atthe viewing surface; and

[0028] applying an electric field of a polarity opposite to the firstpolarity to the cavity, thereby causing the second type of particles toapproach the viewing surface and the cavity to display the secondoptical characteristic at the viewing surface.

BRIEF DESCRIPTION OF DRAWINGS

[0029] The patent or application file contains at least one drawingexecuted in color. Copies of this patent or patent applicationpublication with color drawing(s) will be provided by the Office uponrequest and payment of the necessary fee.

[0030]FIG. 1 of the accompanying drawings is a schematic section showinga white opaque state of a dielectrophoretic display of the presentinvention.

[0031]FIG. 2 is a schematic section similar to FIG. 1 but showing ablack opaque state of the display shown in FIG. 1.

[0032]FIG. 3 is a schematic section similar to FIGS. 1 and 2 but showinga transparent state of the display shown in FIGS. 1 and 2.

[0033] FIGS. 4 to 6 are top plan views through the viewing surface of anexperimental display in the states corresponding to FIGS. 1 to 3respectively.

[0034]FIGS. 7 and 8 illustrate the transition from the white opticalstate of the display shown in FIG. 4 to the transparent state shown inFIG. 6.

[0035] FIGS. 9 to 11 are schematic sections through a microcell displayof the present invention in states corresponding to those of FIGS. 1 to3 respectively.

DETAILED DESCRIPTION

[0036] As already mentioned, this invention provides a dielectrophoreticdisplay comprising a substrate having walls defining at least onecavity, the cavity having a viewing surface and a side wall inclined tothe viewing surface; a suspending fluid contained within the cavity; aplurality of at least one type of particle suspended within thesuspending fluid; and means for applying to the substrate an electricfield effective to cause dielectrophoretic movement of the particles tothe side wall of the cavity.

[0037] References to “viewing surface” and “side wall” herein do notimply that these surfaces are perpendicular to each other, though asubstantially perpendicular arrangement of the two surfaces ispreferred, since when the particles are disposed adjacent the side wallof the cavity, such a perpendicular arrangement minimizes the area ofthe viewing surface occupied by the particles, and hence permits themaximum amount of light to pass through the cavity. The side wall orwalls of the cavity also need not be planar; for example, anencapsulated display of the present invention may use capsules asdescribed in the aforementioned U.S. Pat. No. 6,067,185 having the formof “flattened spheres” (i.e., oblate ellipsoids) with curved side walls.

[0038] In this display, it is necessary that there be a differencebetween the dielectric constant and/or conductivity of the suspendingfluid and that of the substrate to provide the heterogeneous electricfield necessary for dielectrophoresis. Desirably, this difference shouldbe made as large as possible. It may also be advantageous to use acapsule which has a non-circular, and preferably polygonal,cross-section perpendicular to the direction of the applied electricfield since sharply curved regions or corners of the capsule produceincreased field heterogeneity and thus assist the dielectrophoreticmovement of the particles.

[0039] Those skilled in the technology of electrophoretic displays willappreciate that both electrically neutral and electrically chargedparticles can be moved by dielectrophoresis, since dielectrophoreticmovement is dependent upon dipoles induced in the particles by theelectric field and not upon any pre-existing charge on the particles.However, it appears advantageous to use electrically charged particlesin the apparatus and process of the present invention since once theparticles have been moved to the side wall of the capsule bydielectrophoresis, it appears desirable to use normal electrophoreticmovement of the particles to disperse them; it will be appreciated thatsince the heterogeneity of the electric field in an encapsulated displayis due to differences between the properties of the suspending fluid onthe one hand and the capsule wall and surrounding material on the other,there will normally be no way of reversing the high field and low fieldregions in a manner similar to that used in the Batchelder apparatus, sothat if the particle movement caused by dielectrophoresis is to bereversed, some applied force other than dielectrophoresis must be used.

[0040] If electrically charged particles are used in the presentapparatus and process, the particles are of course subject to bothelectrophoretic and dielectrophoretic forces when an electric field isapplied. Typically, electrophoretic movement of particles will be muchmore rapid than dielectrophoretic, so that to ensure that the desireddielectrophoretic movement is not subject to interference fromelectrophoretic movement, it is desirable to reverse the electric fieldat intervals; provided the field is applied for the same amount time ineach direction, the electrophoretic movements will sum to zero, sinceelectrophoretic movement is polarity-sensitive, whereas thedielectrophoretic movements will not sum to zero since dielectrophoreticmovement is polarity-independent.

[0041] The dielectrophoretic movement of the particles in the apparatusand process of the present invention is affected by the material fromwhich the particles are formed, and the size and shape of the particles.Since dielectrophoresis depends upon the induction of dipoles within theparticles, it is desirable to use particles which are highlypolarizable, especially conductive particles such as metals. Forexample, aluminum particles may be used in the present invention. It hasbeen observed experimentally that carbon black particles, which have areasonably high conductivity, have substantially greaterdielectrophoretic mobility than substantially non-conductive titaniaparticles. The particles may also be formed from a doped semiconductor;the type of doping is not critical provided that the particles havesufficient conductivity, but most undoped semiconductors have too low aconductivity to have high dielectrophoretic mobility.

[0042] The induced dipole, and hence the dielectrophoretic movement ofthe particles, is also affected by the size and shape of the particles.Since a large particle allows greater separation between the poles of adipole than a smaller particle, increasing the size of the particleswill increase dielectrophoretic mobility, although of course theparticles should not be made so large as to readily visible when theylie adjacent the side wall of the capsule. For similar reasons, elongateparticles, especially needle-shaped particles, will tend to have ahigher dielectrophoretic mobility than spherical particles of the samevolume. Anisotropically shaped particles may also be useful in thepresent invention.

[0043] There are two main variations of the apparatus and process of thepresent invention. In the first variation, the cavity contains only asingle type of particle in an uncolored suspending fluid. This capsulecan be switched between an “opaque” state, in which the particles aredispersed throughout the suspending fluid, and a “transparent” state, inwhich the particles are moved to a side wall of the capsule so thatlight can pass through the uncolored suspending fluid. The transparentstate need not appear transparent to a viewer; as illustrated in thedrawings and as described in more detail below, a reflector or filterhaving a color different from that of the particles may be placed on theopposed side of the capsule from the viewing surface thereof, so that inthe transparent state a viewer sees the color of the reflector orfilter; in the opaque state the color of the reflector or filter is ofcourse hidden by the dispersed particles.

[0044] In the second variation, the capsules contain two different typesof particles differing in at least one optical characteristic and inelectrophoretic mobility and a suspending fluid which may be colored oruncolored. This capsule can be switched among three states, namely afirst opaque state, in which the first type of particles are visible, asecond opaque state, in which the second type of particles are visible,and a “transparent” state, in which both types of particles are moved toa side wall of the capsule by dielectrophoresis and the color of thesuspending fluid is visible; if, as will typically be the case, thesuspending fluid is uncolored, the transparent state is actuallytransparent and may be used to display the color of a reflector orfilter disposed on the opposed side of the capsule from the viewingsurface thereof, as previously described.

[0045] It will be appreciated that, provided that the desired color canbe seen when a display of the present invention is in a transparentstate, the location of the colored material is essentially irrelevant.Thus, although reference has been made above to a reflector or filter,it is not essential that this reflector or filter be a discrete integer,and color could be provided in any convenient location. Thus, forexample, the colored reflector or filter could be provided by coloring(a) the substrate itself, for example the polymeric film used in amicrocell form of the present display; (b) a material associated withthe substrate, for example a polymeric binder used to retain capsules ina coherent layer in an encapsulated display of the invention, or alamination adhesive layer used to secure the dielectrophoretic layer toa backplane; or (c) the pixel electrodes or another component of abackplane used to drive the display. In principle, in an encapsulateddisplay color could be provided by dyeing the capsule walls themselves,but this does have the disadvantage that in an opaque state of a pixelthe color in the portion of the capsule adjacent the viewing surfacewill affect the color seen at that surface when the pixel is in anopaque state. In some cases, the resultant color shift may beacceptable, or may be minimized, for example by using particles whichhave a color complementary to that of the color caused by the capsulewall. In other cases, color may be provided only on the parts of thecapsule wall lying on the opposed side of the capsule to the viewingsurface, for example by providing a radiation-sensitive color-formingmaterial in the capsule wall and then exposing this color-formingmaterial to radiation effective to bring about the formation of color,this radiation being directed on to the capsule from the side of thedisplay opposite to the viewing surface.

[0046] Color could also be provided from a source separate from thedisplay itself. For example, if a display of the present invention isarranged to operate as a light valve and backlit by projecting light onto a surface on the opposed side of the display from the viewingsurface, color could be provided by imaging an appropriate color filteron to the rear surface of the display.

[0047] Except in cases where it is essential that the colored member belight transmissive, the color may be provided either by dyes orpigments, although the latter are generally preferred since they aretypically more stable against prolonged exposure to radiation, and thustend to provide displays with longer operating lifetimes.

[0048] As already indicated, no special electrode configurations arerequired in the display and process of the present invention; theinvention can be practiced with simple parallel electrodes on opposedsides of the cavity; for example, a multi-pixel display of the inventionusing at least one cavity per pixel could have the conventionalelectrode configuration of a single pixel electrode for each pixel onone side of the cavities and a single common electrode extending acrossall the pixels on the opposed side of the cavities. However, thisinvention does not exclude the possibility that the electrodes might beshaped to enhance the dielectrophoretic effect. It may also be useful touse so-called “z-axis adhesives” (i.e., adhesives having a substantiallygreater conductivity parallel to the thickness of a layer of adhesivethan in the plane of this layer) between one or both of the electrodesand the cavities cf. copending application Ser. No. 60/319,934, filedFeb. 11, 2003, the entire disclosure of which is herein incorporated byreference. In addition, as discussed in detail below with reference tothe drawings, in some embodiments of the invention it may beadvantageous to provide auxiliary electrodes to assist in redispersingthe particles in the suspending fluid after the particles have be drivento the side walls by dielectrophoresis.

[0049] As already mentioned, there are three principal types ofdielectrophoretic displays of the present invention. The first type isthe “classical” encapsulated electrophoretic type as described in theaforementioned E Ink and MIT patents and applications. In this type ofdisplay, the substrate has the form of at least one capsule wall, whichis typically deformable, and formed by depositing a film-formingmaterial around a droplet containing the suspending fluid and thedielectrophoretic particles. The second type is the polymer-dispersedelectrophoretic type in which the substrate comprises a continuous phasesurrounding a plurality of discrete droplets of the suspending fluid.Full details regarding the preparation of this type of display are givenin the aforementioned 2002/0131147. The third type is the microcelldisplay, in which a plurality of cavities or recesses are formed in asubstrate, filled with the suspending fluid and particles and thensealed, either by lamination a cover sheet over the recesses or bypolymerizing a polymerizable species also present in the suspendingfluid.

[0050] The first dielectrophoretic display (generally designed 100) ofthe invention shown in FIGS. 1 to 3 comprises an encapsulateddielectrophoretic medium (generally designated 102) comprising aplurality of capsules 104 (only one of which is shown in FIGS. 1 to 3),each of which contains a suspending liquid 106 and dispersed therein aplurality of a first type of particle 108, which for purposes ofillustration will be assumed to be black. The particles 108 areelectrophoretically mobile and may be formed of carbon black. In thefollowing description, it will be assumed that the particles 108 arepositively charged, although of course negatively charged particlescould also be used if desired. Also suspended in the suspending liquid106 is a plurality of a second type of particle 110, which iselectrophoretically mobile and negatively charged, and may be formed oftitania. (The triangular shape of the particles 108, and the circularshape of the particles 110 are used purely to way of illustration toenable the various types of particles to be distinguished easily in theaccompanying drawings, and in no way correspond to the physical forms ofthe actual particles, which are typically substantially spherical.However, we do not exclude the use of non-spherical particles in thepresent displays.) The display 100 further comprises a common,transparent front electrode 112, which forms a viewing surface throughwhich an observer views the display 100, and a plurality of discreterear electrodes 114, each of which defines one pixel of the display 100(only one rear electrode 114 is shown in FIGS. 1 to 3). (The frontelectrode 112 is typically provided on a support member which alsoprovides mechanical protection for the display 100 but for simplicitythis support member is omitted from FIGS. 1 to 3.) For ease ofillustration and comprehension, FIGS. 1 to 3 show only a singlemicrocapsule forming the pixel defined by rear electrode 114, althoughin practice a large number (20 or more) microcapsules are normally usedfor each pixel. The rear electrodes 114 are mounted upon a substrate116, which contains areas of differing color, as described in moredetail below with reference to FIGS. 4 to 8.

[0051] Typically the liquid 106 is uncolored (i.e., essentiallytransparent), although some color may be present therein to adjust theoptical properties of the various states of the display. FIG. 1 showsthe display 100 with the front electrode 112 positively charged relativeto the rear electrode 114 of the illustrated pixel. The positivelycharged particles 108 are held electrostatically adjacent the rearelectrode 114, while the negatively charged particles 110 are heldelectrostatically against the front electrode 112. Accordingly, anobserver viewing the display 100 through the front electrode 112 sees awhite pixel, since the white particles 110 are visible and hide theblack particles 108.

[0052]FIG. 2 shows the display 100 with the front electrode 112negatively charged relative to the rear electrode 114 of the illustratedpixel. The positively charged particles 108 are now electrostaticallyattracted to the negative front electrode 112, while the negativelycharged particles 110 are electrostatically attracted to the positiverear electrode 114. Accordingly, the particles 108 move adjacent thefront electrode 112, and the pixel displays the black color of theparticles 108, which hide the white particles 110.

[0053]FIG. 3 shows the display 100 after application of an alternatingelectric field between the front and rear electrodes 112 and 114respectively. The application of the alternating electric field causesdielectrophoretic movement of both types of particles 108 and 110 to theside walls of the capsule 104, thus leaving the major portion of thearea of the capsule 104 essentially transparent. Accordingly, the pixeldisplays the color of the substrate 116.

[0054] To redisperse the particles 108 and 110 uniformly throughout thesuspending liquid 106 from their positions shown in FIG. 3, a series ofshort direct current voltages of alternating polarity is applied betweenthe front and rear electrodes 112 and 114, thereby causing the particles108 and 110 to oscillate within the suspending liquid 106; thisoscillation causes the particles 108 and 110 to gradually redispersethroughout the liquid 106. Application of a longer direct current pulseof appropriate polarity will then cause the pixel to assume the stateshown in FIG. 1 or 2 depending upon the polarity of the longer pulse.

[0055] In FIGS. 1 to 3, the capsules 104 are illustrated as being ofsubstantially prismatic form, having a width (parallel to the planes ofthe electrodes) significantly greater than their height (perpendicularto these planes). This prismatic shape of the capsules 104 is deliberatesince it provides the capsules with side walls which extend essentiallyperpendicular to the viewing surface of the display, thus minimizing theproportion of the area of the capsule 104 which is occupied by theparticles 108 and 110 in the transparent state shown in FIG. 3. Also, ifthe capsules 104 were essentially spherical, in the black state shown inFIG. 2, the particles 108 would tend to gather in the highest part ofthe capsule, in a limited area centered directly above the center of thecapsule. The color seen by the observer would then be essentially theaverage of this central black area and a white or colored annulussurrounding this central area, where either the white particles 110 orthe substrate 116 would be visible. Thus, even in this supposedly blackstate, the observer would see a grayish color rather than a pure black,and the contrast between the two extreme optical states of the pixelwould be correspondingly limited. In contrast, with the prismatic formof microcapsule shown in FIGS. 1 and 2, the particles 108 coveressentially the entire cross-section of the capsule so that no, or atleast very little, white or other colored area is visible, and thecontrast between the extreme optical states of the capsule is enhanced.For further discussion on this point, and on the desirability ofachieving close-packing of the capsules within the electrophoreticlayer, the reader is referred to the aforementioned U.S. Pat. No.6,067,185. Also, as described in the aforementioned E Ink and MITpatents and applications, to provide mechanical integrity to thedielectrophoretic medium, the capsules 104 are normally embedded withina solid binder, but this binder is omitted from FIGS. 1 to 3 for ease ofillustration.

[0056]FIGS. 4, 5 and 6 of the accompanying drawings illustrate the whiteopaque, black opaque and transparent optical states of an experimentaldisplay of the present invention substantially as described above withreference to FIGS. 1 to 3 and comprising a plurality of capsules, eachof which contains carbon black and white titania particles bearingcharges of opposite polarity in a colorless suspending fluid. Thedisplay was prepared substantially as described in the aforementioned2003/0137717 by encapsulating a hydrocarbon suspending fluid containingthe carbon black and titania particles in a gelatin/acacia capsule wall,mixing the resultant capsules with a polymeric binder, coating thecapsule/binder mixture on to an indium tin oxide (ITO) coated surface ofa polymeric film to provide a single layer of capsules covering thefilm, and laminating the resultant film to a backplane. For purposes ofillustration, the display shown in FIGS. 4, 5 and 6 was formed as asingle pixel with the transparent front electrode forming the viewingsurface of the display, and the backplane (actually a single rearelectrode) disposed adjacent a multicolored reflector.

[0057]FIG. 4 shows the display in its first, white opaque statecorresponding to that of FIG. 1, with the white particles moved byelectrophoresis and lying adjacent the viewing surface of the display,so that the white particles hide both the black particles and themulticolored reflector, and the display appears white. Similarly, FIG. 5shows the display in its second, black opaque state corresponding tothat of FIG. 2, with the black particles moved by electrophoresis andlying adjacent the viewing surface of the display, so that the blackparticles hide both the white particles and the multicolored reflector,and the display appears black. FIG. 6 shows the display in a transparentstate corresponding to that of FIG. 3 caused by applying a square wavewith a frequency of 60 Hz and an amplitude of 90V until no furtherchange was visible in the display (approximately 150 seconds). Theapplication of this square wave caused both the black and whiteparticles to move dielectrophoretically to the side walls of thecapsules, thus causing the multicolored reflector to be visible throughthe uncolored suspending fluid. Thus, a display of the type shown inFIGS. 1 to 6 can display three different colors, which eases theproblems of building a full color electro-optic display.

[0058]FIGS. 7 and 8 illustrate the transition from the white opaquestate shown in FIG. 4 to the transparent state shown in FIG. 6; FIG. 7shows the display after application of the aforementioned square wavefor 10 seconds, while FIG. 8 shows the display after application of thesquare wave for 30 seconds. It will be seen from FIGS. 6, 7 and 8 thatdevelopment of the transparent state occurs gradually as more and moreparticles are moved to the side walls of the capsules. In FIG. 7, themulticolored reflector is just becoming visible, while in FIG. 8 thisreflector is more visible but much less clear than in the finaltransparent state shown in FIG. 6.

[0059] FIGS. 9 to 11 show schematic sections, similar to those of FIGS.1 to 3 respectively, of one pixel of a microcell display (generallydesignated 900) of the present invention. The microcell display 900 usesessentially the same dielectrophoretic medium, comprising a suspendingliquid 106 with carbon black particles 108 and white titania particlessuspended therein, as the encapsulated display 100 shown in FIGS. 1 to3; however, the form of substrate used in the display 900 differssubstantially from that of the display 100. In the display 900, thesubstrate comprises a base member 120 and a plurality of side walls 122extending perpendicular to the base member 120 and forming a pluralityof microcells in which are confined the liquid 106 and the particles 108and 110. The lower faces (as illustrated in FIGS. 9 to 11) of themicrocells are closed by closure walls 124, which are formed byradiation polymerization of a polymerizable species originally presentin the liquid 106; see the aforementioned International ApplicationPublication No. WO 02/01281, and published U.S. Application No.2002/0075556. The display 900 further comprises a front electrode 112, arear or pixel electrode 114 and a colored substrate 116 all of which areessentially identical to the corresponding integers in FIG. 1. (In thesame way as in FIGS. 1 to 3, for simplicity FIGS. 9 to 11 are drawn asif there is only a single microcell to the pixel defined by theelectrode 114 although in practice a single pixel may comprise multiplemicrocells.) The display 900 also comprises auxiliary electrodesembedded within the side walls 122 and a protective layer 126 coveringthe front electrode 112.

[0060] As shown in FIGS. 9 to 11, the microcell display 900 operates ina manner very similar to the encapsulated display 100 shown in FIGS. 1to 3. FIG. 9 shows the display 900 with the front electrode 112positively charged relative to the rear electrode 114 of the illustratedpixel. The positively charged particles 108 are held electrostaticallyadjacent the rear electrode 114, while the negatively charged particles110 are held electrostatically against the front electrode 112.Accordingly, an observer viewing the display 100 through the frontelectrode 112 sees a white pixel, since the white particles 110 arevisible and hide the black particles 108.

[0061]FIG. 10 shows the display 900 with the front electrode 112negatively charged relative to the rear electrode 114 of the illustratedpixel. The positively charged particles 108 are now electrostaticallyattracted to the negative front electrode 112, while the negativelycharged particles 110 are electrostatically attracted to the positiverear electrode 114. Accordingly, the particles 108 move adjacent thefront electrode 112, and the pixel displays the black color of theparticles 108, which hide the white particles 110.

[0062]FIG. 11 shows the display 900 after application of an alternatingelectric field between the front and rear electrodes 112 and 114respectively. The application of the alternating electric field causesdielectrophoretic movement of both types of particles 108 and 110 to theside walls of the microcell, thus leaving the major portion of the areaof the microcell essentially transparent. Accordingly, the pixeldisplays the color of the substrate 116.

[0063] Redispersion of the particles 108 and 110 from the transparentstate of the display 900 shown in FIG. 11 may be effected in the sameway as described above for the display 100. However, the auxiliaryelectrodes 126 are provided to assist in such redispersion. Theauxiliary electrodes run the full width of the display (which is assumedto be perpendicular to the plane of FIGS. 9 to 11), i.e., each auxiliaryelectrode is associated with a full row of microcells, and the auxiliaryelectrodes are connected to a voltage source which, when activated,applies voltages of opposed polarities to alternate auxiliary electrodes126. By applying a series of voltage pulses of alternating polarity tothe auxiliary electrodes 126, an electric field is created in theleft-right direction in FIGS. 9 to 11, which greatly assists isredispersing all the particles 108 and 110 throughout the displayuniformly within the liquid 106. Voltage pulses of alternating polaritymay also be applied to the electrodes 112 and 114 as previouslydescribed to further assist in redispersing the particles 108 and 110.

[0064] It will be appreciated that the present invention need not makeuse of a colored reflector behind the capsules but may be used toprovide backlit displays, variable transmission windows and transparentdisplays; indeed, the present invention may be useful in any where lightmodulation is desired.

[0065] Those skilled in the display art will appreciate that numerouschanges, improvements and modifications can be made in the preferredembodiments of the invention already described without departing fromthe scope of the invention. Accordingly, the whole of the foregoingdescription is intended to be construed in an illustrative and not in alimitative sense.

1 a dielectrophoretic display comprising: a substrate having wallsdefining at least one cavity, the cavity having a viewing surface and aside wall inclined to the viewing surface; a suspending fluid containedwithin the cavity; a plurality of at least one type of particlesuspended within the suspending fluid; and means for applying to thesubstrate an electric field effective to cause dielectrophoreticmovement of the particles to the side wall of the cavity. 2 Adielectrophoretic display according to claim 1 wherein the suspendingfluid is substantially uncolored, and has suspended therein only asingle type of particle. 3 A dielectrophoretic display according toclaim 1 wherein at least some of the at least one type of particle areelectrically charged. 4 A dielectrophoretic display according to claim 3wherein the suspending fluid has suspended therein a first type ofparticle having a first optical characteristic and a firstelectrophoretic mobility, and a second type of particle having a secondoptical characteristic different from the first optical characteristicand a second electrophoretic mobility different from the firstelectrophoretic mobility. 5 A dielectrophoretic display according toclaim 4 wherein the first and second electrophoretic mobilities differin sign, so that the first and second types of particles move in opposeddirections in an electric field. 6 A dielectrophoretic display accordingto claim 4 wherein the suspending fluid is substantially uncolored. 7 Adielectrophoretic display according to claim 4 further comprising abacking member disposed on the opposed side of the cavity from theviewing surface, at least part of the backing member having a thirdoptical characteristic different from the first and second opticalcharacteristics. 8 A dielectrophoretic display according to claim 7wherein the backing member comprises areas having third and fourthoptical characteristics different from each other and from the first andsecond optical characteristics. 9 A dielectrophoretic display accordingto claim 7 wherein the backing member comprises areas having red, greenand blue or yellow, cyan and magenta colors. 10 A dielectrophoreticdisplay according to claim 7 wherein the first and second opticalcharacteristics comprise black and white colors. 11 A dielectrophoreticdisplay according to claim 1 wherein the cavity has a non-circularcross-section as seen from the viewing surface. 12 A dielectrophoreticdisplay according to claim 11 wherein the cavity has a polygonalcross-section as seen from the viewing surface. 13 A dielectrophoreticdisplay according to claim 1 wherein the at least one type of particleis formed from an electrically conductive material. 14 Adielectrophoretic display according to claim 13 wherein the at least onetype of particle is formed from a metal or carbon black. 15 Adielectrophoretic display according to claim 1 wherein the at least onetype of particle is formed from a doped semiconductor. 16 Adielectrophoretic display according to claim 1 wherein the substratecomprises at least one capsule wall so that the dielectrophoreticdisplay comprises at least one capsule. 17 A dielectrophoretic displayaccording to claim 16 comprising a plurality of capsules, the capsulesbeing arranged in a single layer. 18 A dielectrophoretic displayaccording to claim 1 wherein the substrate comprises a continuous phasesurrounding a plurality of discrete droplets of the suspending fluidhaving the at least one type of particle suspended therein. 19 Adielectrophoretic display according to claim 1 wherein the substratecomprises a substantially rigid material having the at least one cavityformed therein, the substrate further comprising at least one covermember closing the at least one cavity. 20 A process for operating adielectrophoretic display, the process comprising: providing a substratehaving walls defining at least one cavity, the cavity having a viewingsurface and a side wall inclined to the viewing surface; a suspendingfluid contained within the cavity; and a plurality of at least one typeof particle suspended within the suspending fluid; and applying to thesubstrate an electric field effective to cause dielectrophoreticmovement of the particles to the side wall of the cavity. 21 A processaccording to claim 20 wherein the electric field is an alternatingelectric field. 22 A process according to claim 20 wherein at least someof the at least one type of particle are electrically charged. 23 Aprocess according to claim 22 wherein the suspending fluid has suspendedtherein a first type of particle having a first optical characteristicand a first electrophoretic mobility, and a second type of particlehaving a second optical characteristic different from the first opticalcharacteristic and a second electrophoretic mobility different from thefirst electrophoretic mobility. 24 A process according to claim 23wherein the first and second electrophoretic mobilities differ in sign,so that the first and second types of particles move in opposeddirections in an electric field. 25 A process according to claim 24further comprising: applying an electric field of a first polarity tothe cavity, thereby causing the first type of particles to approach theviewing surface and the cavity to display the first opticalcharacteristic at the viewing surface; and applying an electric field ofa polarity opposite to the first polarity to the cavity, thereby causingthe second type of particles to approach the viewing surface and thecavity to display the second optical characteristic at the viewingsurface. 26 A process according to claim 23 further comprising providinga backing member disposed on the opposed side of the cavity from theviewing surface, at least part of the backing member having a thirdoptical characteristic different from the first and second opticalcharacteristics. 27 A process according to claim 26 wherein the backingmember comprises areas having third and fourth optical characteristicdifferent from each other and from the first and second opticalcharacteristics. 28 A process according to claim 20 wherein the at leastone type of particle is formed from an electrically conductive material.29 A process according to claim 28 wherein the at least one type ofparticle is formed from a metal or carbon black. 30 A process accordingto claim 28 wherein the at least one type of particle is formed from adoped semiconductor. 31 A process according to claim 20 wherein thesubstrate comprises at least one capsule wall so that thedielectrophoretic display comprises at least one capsule. 32 A processaccording to claim 20 wherein the substrate comprises a plurality ofcapsules, the capsules being arranged in a single layer. 33 A processaccording to claim 20 wherein the substrate comprises a continuous phasesurrounding a plurality of discrete droplets of the suspending fluidhaving the at least one type of particle suspended therein. 34 A processaccording to claim 20 wherein the substrate comprises a substantiallyrigid material having the at least one cavity formed therein, thesubstrate further comprising at least one cover member closing the atleast one cavity.